1. Field of the Invention
This invention pertains to a control system for use with pressure-driven loads, such as hydraulic or pneumatic motors or pistons. More particularly, this invention pertains to a control system which allows such loads to be reversed and which further allows the rate of operation of the loads to be controlled. Most specifically, this invention pertains to a control system for such loads which is designed for use with an adjustable-pressure pump which is non-reversible.
2. Background of the Prior Art
This type of control system is known and indeed in common use, especially when the pressure-driven load is a reversible hydraulic motor or piston. In known pressure-control systems, an operator can manually reverse the load by operating suitably connected valves which can reverse the direction of pressurized hydraulic fluid that passes through the load. Moreover, known control systems allow the rate of operation of such a load to be varied by adjusting a constriction valve which is in series with the load. Thus, using known control systems, both the direction of operation and the speed of operation of such a load may be varied by an operator.
However, in the event that an operator decides to abruptly change the direction of operation of such a load, or decides to abruptly change its operating speed, the load may be subjected to pressure shock. For example, in a hydraulic machine tool, an abrupt reversal of a hydraulic motor which is carrying a work piece will subject the motor to an immediate pressure shock when the quickly reversed hydraulic fluid is suddenly routed through the motor in an opposite direction. This pressure shock, if repeated, can rapidly damage and totally destroy the motor. Thus, it is desirable to provide a control system which does not subject such a motor or a load to such a pressure shock, even if an operator abruptly changes the direction in which the load is supposed to operate or even if he abruptly alters the speed at which it is supposed to operate.